Several people rely on many portable devices, such as cell phones or digital audio players. These devices typically require the use of batteries, and the batteries must either be replaced or recharged on a regular basis. Batteries typically do not last as long as users would like. Furthermore, the voltage output of batteries tend to decay as they are used up, resulting in reduced output from devices, such as flash lights.
In LED (light emitting diode) flashlights, change in battery voltage is a problem because the LED is bright to start with but the brightness rapidly drops off as the battery voltage decreases. LED flashlights powered by batteries, such as alkaline batteries, typically have a long “tail” period when the light output is low. The battery capacity is wasted because the light output drops below a given “usable” threshold. For example, the low output must be endured or the batteries must be prematurely discarded. The resulting volumetric energy density of these alkaline-battery-based flashlights is often very low. While rechargeable batteries can be used, rechargeable batteries can have a high self-discharge rate, meaning that the batteries discharge over time regardless of whether the flashlight is turned on or not. When the user wants to use the flashlight, the flashlight may not work if the rechargeable battery has not been recently recharged or if the self-discharge rate is high. This is at best an inconvenience but could be a life or death problem in an emergency situation. Furthermore, rechargeable batteries have limited cycle life. As the number of recharging cycles increase, battery capacity decreases. This can be frustrating for the user because run time of the flashlight will decrease. Replacing custom rechargeable batteries can also be expensive.